The field of the invention relates to control of a motor vehicle drivetrain during an automatic transmission shift.
Automatic transmissions typically include multiple clutches coupled to planetary gears for gear ratio selection. During a shift, the friction elements of a selected clutch gradually become engaged for transferring torque through a desired gear ratio. As is well known, initial torque transfer occurs during a slippage phase of the friction elements. These elements are designed with sufficient mass and surface area for dissipating the friction heat generated during the slippage phase.
The inventors herein have recognized that recent developments in high speed, high power engines, such as four valve per cylinder engines, result in more heat than conventional friction elements can adequately dissipate during automatic transmission shifts. New transmissions with larger friction elements could be designed to dissipate this excessive heat, however, the additional packaging area required would be prohibitive.
Although engine control strategies are known to reduce shift shock, these strategies are not suitable to address the problem of excessive friction heat build up during high speed shifts utilizing high output engines. For example, U.S. Pat. No. 4,403,527 issued to Mohl et al discloses a control unit for retarding ignition spark during a shift to reduce undesired jolts and resulting wear of the friction elements commonly known as shift shock. Reducing fuel supplied by electronic fuel injectors to reduce shift shock is also disclosed. U.S. Pat. No. 4,800,781 issued to Yasue et al also discloses retarding ignition timing to reduce shift shock and U.S. Pat. No. 4,226,141 also discloses reducing fuel to reduce shift shock.
These prior approaches however do not, and can not, address the problem of excessive friction heat generated during high speed shifts utilizing a high output engine. More specifically, spark timing can only reduce engine torque output by approximately 25% (see The Internal Combustion Engine in Theory and Practice, Volume 1, MIT Press, 1980, by Taylor) whereas a reduction in excess of 50% may be required to couple a high output engine to a conventional transmission. Further, the incomplete combustion resulting from maximum spark retard may result in excessive exhaust heat and damage to the catalytic converter (see U.S. Pat. No. 4,800,781 issued to Yasue et al at lines 36-55 of column 1). Fuel injection control has also been found to be unsatisfactory due to the time delay for an air/fuel charge to be inducted through an intake manifold into the engine.